Ask the Brains - August 2008

Can one neuron release more than one neurotransmitter?—Marvin Shrewsbury, Wailuku, Hawaii

Rebecca Seal, a neuroscientist at the University of California, San Francisco, replies:

WHEN DISCUSSING neurotransmitters, most people think of the classical neurotransmitters, such as dopamine and serotonin—the primary chemical messengers used by neurons to communicate with one another and with other types of cells. In the early 20th century physiologist Sir Henry Dale hypothesized that an individual neuron releases the same classical neurotransmitter from all its axons, the spindly branches that jut out from the main cell body. Another prominent scientist of the time, Sir John Eccles, restated Dale’s principle to also mean that a neuron releases only one neurotransmitter. From that point on, the concept of “one neuron, one transmitter” became widely accepted.

Neuroscientists now know, however, that it is common for neurons to release a classical transmitter with another type of messenger, such as a gas (nitric oxide, for instance) or a neuropeptide (a small protein that can act as a transmitter). With the aid of new techniques for manipulating and imaging neurons, researchers have found that a number of neurons communicate using more than one classical neurotransmitter. Indeed, some of our auditory neurons simultaneously release three different classical transmitters during a brief period in development.

So we see that “one neuron, one transmitter” is a bit too simplistic. But what about the original principle put forth by Dale that all axonal branches of a neuron release the same transmitter? There now appear to be at least a few exceptions to this principle. Motor neurons, which are important for voluntary muscle movements, have long been known to release acetylcholine onto both muscle cells in the body and neurons in the spinal cord. Recent studies show, however, that motor neurons also release a second transmitter, glutamate. Remarkably, they appear to release glutamate only onto neurons in the spinal cord and not onto muscle cells—in other words, certain branches of a single neuron release glutamate, and others do not.

A next step in neurotransmitter research will be to understand how the release of more than one messenger ­affects the function of the neural cir­-cuit and the organism as a whole. The fundamental question you ask has led to nearly a century of fascinating research, and it will continue to be an active and exciting area of investigation.

Why is it comforting to discuss problems with others?—Celine Joiris, via e-mail

Dinah Miller, a psychiatrist in private practice in Baltimore and a part-time faculty member at Johns Hopkins University, explains:

WHEN PEOPLE seek comfort in talking, they may be looking to unburden themselves of a secret or seeking validation for their beliefs. Sometimes they want reassurance that nothing is terribly wrong with them. Psychotherapy, or talk therapy, has traditionally been part of the treatment for mental disorders; the process of talking is itself instrumental in alleviating such problems.

It is refreshing that your question asks why talking is comforting and not why talking is curative. It can be difficult to assess what features of psychotherapy are healing; it is easier to break down the components of why a patient may feel comforted. The question does not specifically address the talking of psychotherapy as opposed to the talking that occurs between friends or in a support group, but many helpful elements are shared by all these settings.

The primary factor that yields comfort is the relationship between the distressed person and the listener. It is vitally important that the speaker feels heard and that he or she has the opportunity to discuss a situation in an open and accepting environment. Often people seek out listeners who have been through the same experience and can offer true empathy. For the sake of completeness, I should say that talking does not comfort everyone.